Parasites of Arthropods and Helminths

 A number of disease-causing multicellular organisms are also studied using the same microscopic and immunological techniques that are used to study microorganisms and viruses. As a result, they are included here. Most of the medically important multicellular parasites fall into one of two groups: arthropods and helminths.  

The arthropods are more highly advanced on the evolutionary scale and include the insects, ticks, lice, and mites.

 Their main medical importance is that they serve as vectors that may transmit microorganisms and viruses to humans. The helminths, which include the nematodes (roundworms), cestodes (tapeworms), and the trematodes (flukes), are more primitive animals. In only a few instances do they transmit microbial infections to their host animal. Instead, they cause disease by invading the host’s tissues or robbing it of nutrients.

Most multicellular parasites have been well controlled in the industrialized nations, but they still cause death and misery to many millions in the economically underdeveloped areas of the world.

 Our need to know about these problems has come about because more people are traveling farther, more people are moving from one place to another, and more goods are being exchanged worldwide. A clear example of this occurred in New York City in the summer of 1999 when West Nile fever was contracted by a number of people.

 At least 61 persons suffered serious disease and seven people died. A significant number of crows died at the same time and were found to be carrying the disease. In addition to birds and people, horses, cats, and dogs were also found to carry the virus. It is not clear how the virus arrived in New York City, but it perhaps could have been carried by a traveler from Africa, West Asia, or the Middle East, where it is commonly found. It could possibly have been brought by an imported bird from the same areas. Worldwide travel makes us more vulnerable to diseases from other parts of the world

In addition, worldwide climatic conditions are changing and bringing increases in certain insect populations to areas that were previously free of them. As a result, more cases of multi- cellular parasitic infections are being seen by physicians in the United States than previously.

ascomycota characteristics,ascomycota life cycle,ascomycota classification,ascomycota reproduction

 Members of Ascomycota, or ascomycetes, commonly known as sac fungi, are named for their characteristic reproductive structure, the saclikeascus (pl., asci; Greek askos, sac). 

Ascomycetes are  ecologically  important  in  freshwater, marine,and terrestrial habitats because they degrade many chemically stable organic compounds, including lignin, cellulose, and col­lagen.

 Many species are quite familiar and economically im­portant. For example, most of the red, brown, and blue-green molds that cause food spoilage are ascomycetes.

 The powdery  mildews  that  attack  plant  leaves  and  the  fungi  that cause chestnut blight and Dutchelm disease are ascomycetes. Many yeasts as well as edible morels and truffles are also asco­ mycetes.

 The pink bread mold Neurospora crassa is an impor­tant research tool in genetics and biochemistry.  A  new ascomycete genus, Archaeorhizomyces, was described in 2011.

 The  group  is  globally  distributed and is  commonly  found in  as­sociation with the roots of pine trees but does not appear to be mycorrhizal, as it can be grown in pure culture. Like Neuros­ pora and Aspergillus, Archaeorhizomyces spp. are filamentous, producing slender hyphae.

Some ascomycetes are yeasts, while others have a life cycle that alternates between yeast and filamentous forms. The life cycle of the yeast Saccharomyces cerevisiae, commonly known ashas been invaluable for understanding the loss of cell cycle con­trol that occurs in cancerous cells.

Filamentous ascomycetes form septate hyphae. Asexual re­ production is common and is associated with the production  of conidia . 

Sexual reproduction involves ascus for­mation, with each ascus usually bearing eight haploid asco­ spores, although some species can produce over 1,000. Such hyphae are said to be ascogenous.

 Mating starts  when  two strains  of  opposite  mating  types  form  ascogenous  hyphae into which  pairs of  nuclei migrate.

  One nucleus of each pair originates from a "male" mycelium (antheridium) or cell and the other from a "female" organ or cell (ascogonium) that has fused with it. As the ascogenous hyphae grow, the paired nuclei divide so that there is one pair of nuclei in each cell.

After the ascogenous hyphae have matured, nuclear fusion occurs at the hyphal tips in the ascus mother cells. The diploid zygote nucleus then undergoes meiosis, and the resulting four haploid nuclei divide mitotically again to produce a row of eight brewer's or baker's yeast, is well understood . S. ceretfisiae alternates between haploid and diploid states. As long as nutrients remain plentiful, haploid and diploid cells undergo mitosis to produce haploid and diploid daughter cells, respec­ tively. 

Each daughter cell leaves a scar on the mother cell as it separates, and daughter cells bud only from unscarred regions of the cell wall. When a mother cell has no more unscarred cell wall re­ maining, it can no  longer  repro­ duce and will senesce (die).

 When nutrients are limited, diploid S. ceretfisiae cells undergo  meio- sis to produce four haploid cells that remain bound within a com­ mon cell wall, the ascus. Upon theaddition of nutrients, two haploid cells of opposite mating types  come into contact and fuse to cre­ ate a diploid. 

Typically only cells of opposite mating types can fuse; this process is tightly regulated by the action of pheromones.

S. ceretfisiae is a valuable model   organism.   Research  on this  organism  has  revealed the importance of many cellular pro­ cesses. For instance, it is a favorite model system for studying cell cycling and the events during mi­ tosis. 

This research is critical not only for our understanding of normal cell division, but it alsonuclei in each developing ascus. These nuclei are walled off from one another. Thousands of asci may be packed together in a cup- or flask-shaped fruiting body called an ascocarp.  

When the ascospores mature, they often are released from the asci with great force. If the mature ascocarp is jarred, it may appear to belch puffs of "smoke" consisting of thousands of ascospores. Upon reaching a suitable environment, the ascospores germi­ nate and start the cycle anew.

Several Aspergillus species are noteworthy. A. fumigatus is ubiquitous in the environment, commonly found in homes and the workplace.

 It is known to trigger allergic responses and is im­ plicated in the increased incidence in severe asthma and sinusitis. 

It is also pathogenic, infecting immunocompromised individuals with a mortality rate of nearly 50%. A. nidulans is a model organ­ ism used to study questions of eukaryotic cell and developmental biology. 

A. oryzae is used in the production of traditional fer­ mented foods and beverages in Japan, including saki and soy sauce. Because A. oryzae secretes many industrially useful pro­ teins and can be genetically manipulated, it has become an impor­tant organism in biotechnology.   

Many ascomycetes are parasites of higher plants. Claviceps purpurea parasitizes rye and other grasses, causing the plant dis­ ease ergot.

 Ergotism, the toxic condition in hu­mans and animals that eat grain infected with the fungus, is often accompanied by gangrene, psychotic delusions, nervous spasms, abortion, and convulsions.

 During the Middle Ages, ergotism, then known as St. Anthony's fire, killed thousands of people. For example, over 40,000 deaths from ergot poisoning were recorded in France in the year 943.

 It has been suggested that the widespread accusations of witchcraft in Salem Village and other New England communities in the 1690s may have resulted from outbreaks of ergotism. The pharmacological activities are due to an active ingredient, lysergic acid diethyl­ amide (LSD).

 In controlled dosages, other active compounds can be used to induce labor, lower blood pressure, and ease migraine headaches.

Although conidia are the major form of dissemination, some filamentous fungi also produce sclerotia. Sclerotia are compact masses of hyphae that can survive the winter.

 In the spring they germinate to produce more hyphae or conidia. These structures confer a competitive advantage to the fungi that produce them.

 For instance, some species of Aspergillus that infect plants form sclerotia to remain viable in the soil, where they can take advan­ tage of nutrient resources when the temperature rises.

 Most fungal pathogens that infect animals, including hu­ mans, are ascomycetes . Many are opportunistic pathogens such as those in the genera Candida, Blastomyces,

and Histoplasma. In addition, the cause of"sick building syndrome;'

Stachybotrys chartarum is also an ascomycete .

 Finally, the Aspergillus toxins known as aflatoxins are an important cause of  food  contamination.  Exposure to aflatoxins can result in liver cancer. 

 

Zygomycota,zygomycota reproduction cycle,zygomycota reproduction,zygomycota definition

Zygomycota contains fungi informally called zygomycetes.

Most live on decaying plant and animal matter in the soil; a few are parasites of plants, insects, and animals, including humans.

 The hyphae of zygomycetes are coenocytic, with many haploid nuclei. Asexual spores develop in sporangia at the tips of aerial hyphae and are usually wind dispersed. Sexual reproduction produces tough, thick-walled zygotes called zygospores that can remain dormant when the environment is too harsh for growth of the fungus.

The mold Rhizopus stolonifer is a common member of this division. This fungus grows on the surface of moist, carbohydrate-rich foods, such as breads, fruits, and vegetables. 

Hyphae called rhizoids extend into the bread and absorb nutri­ents. Other hyphae (stolons) become erect, then arch back into the substratum, forming new rhizoids.

Still others remain erect and produce at their tips asexual sporangia filled with black spores, giving the mold its characteristic color. Each spore, when liberated, can germinate to start a new mycelium.

Rhizopus spp.  usually  reproduce asexually,  but if  food be­comes scarce or environmental conditions unfavorable, sexual reproduction occurs . Sexual reproduction requires compatible strains of opposite mating types. When the two mat­ ing strains are close, each produces a different hormone, called a pheromone, that causes their hyphae to form projections called progametangia; these mature into gametangia.

 After fusion of the gametangia, the nuclei of the two gametes fuse, forming a zygote.

 The zygote devel­ops a thick, rough, black coat and be­ comes a dormant zygospore.

Meiosis often occurs at the time of germination; the zygospore then splits open and pro­duces a hypha that bears an asexual spo­rangium to begin the cycle again.

One member of the genus Rhizopus is important because it is involved in the rice disease known as seedling blight. 

If one considers that rice feeds more people on Earth than any other crop, the impli­ cations of this disease are obvious. it was thought that the fungus secreted a toxin that kills rice seedlings, so scientists set about isolating the toxin and the genes that produce it. 

Much to everyone's sur­prise, an a-proteobacterium, Burkholde­ ria sp. found growing within the fungus produces the toxin.

Zygomycetes also contribute to hu­man welfare. 

 For example, one species of Rhizopus is used in Indonesia to produce a food called tempeh from boiled, skinless soybeans. 

Another zygomycete (Mucor spp.) is used with soybeans in Asia to make a curd called sufu.

Others are em­ployed in the commercial preparation of some anesthetics, birth control agents, in- dustrial alcohols, meat tenderizers,  and the yellow coloring used in margarine and butter substitutes.

 

Lysosomes, lysosomes function,lysosomes function in animal cell,lysosomes structure

 Lysosomes are found in animal cells. 

They are roughly spherical, are enclosed in a single membrane, and average about 500 nm in diameter but range from 50 nm to several 11m in size. 

 They are involved in intracellular digestion and contain the enzymes needed to digest all types of macromolecules. 

These enzymes, called hydrolases, catalyze the hydrolysis of molecules and func­ tionbest under slightly acidic conditions (usually around pH 3.5to 5.0). 

 Lysosomes maintain an acidic environment by pumping protons into their interior.

 

Lysosome-like organelles are found in fungal and protist cells, where they are usually called vacuoles, phagocytic vacu­ oles, or food vacuoles.

They function in intracellular digestion, but many have other functions, including storage of calciumions, phosphate, and amino acids. These organelles are compo­nents of the endocytic pathways observed in protists and fungi, just as lysosomes are part of the endocytic pathways observed in animal cells.